Sorry, we’re a bit late with this. This Time interview with Shigeru Miyamoto says the Nintendo dev boss “is now working on a music game. It’s due later this year, even if it takes a few Bowser-like outbursts to pull off.”
The game is most likely the previously announced Wii Music, which has already been confirmed for a 2008 release. Still, nice to kno he hasn’t forgotten about it, eh?